The present invention relates to elastic coverings made of an elastomeric material and the use of these coverings to close off or seal the nozzle outlet area on the ink printing head of an ink ejection system used in ink jet printing.
The ink printing head of an ink ejection system is filled with ink for printing purposes and filled with ink or another fluid, such as a neutral, aqueous liquid for purposes of transportation and storage. The nozzle outlet area must be closed off or sealed from the atmosphere during relatively long periods of inactivity, during transportation, and during storage. For closing off the nozzle outlet area and the nozzle orifices emanating therefrom, use should be made not only of the elastic characteristic of elastomeric materials, but also, particularly, the viscoelastic (flow) behavior of these elastomers for filling up exceedingly small cavities and interstices in the zone of the closed nozzle outlet area. The nozzle zone of ink printing heads is sensitive to contamination caused, for example, by paper fibers and dust accumulations. Contamination in the nozzle zone leads, with secondary effects, to an altered nozzle geometry, so that droplets emitted from the nozzle are greatly affected in size, shape, direction, and velocity. In the case of a relatively long inactivity between periods of printing, during transport and storage of the ink printing heads or of the printing units equipped with such printing heads, it is, furthermore, necessary to avoid an excessive thickening of the ink in the aperture regions of the nozzles and leakage of the ink or corresponding fluid by shock during transportation, or the drawing of air into the printing head together with the leakage of ink or fluid.
German Application DOS 2,702,663, discloses an elastic covering for the closing of nozzles of an ink printing head filled with an aqueous liquid, in which the covering consists of an elastomeric material, especially of a silicone rubber. In this arrangement, the viscoelastic (flow) behavior of these elastomeric materials is used to fill up exceedingly small cavities and interstices in the region of the closed nozzle outlet area. In addition to these intentionally emphasized properties, however, these silicone elastomeric materials also exhibit disadvantages due to their inherent properties caused by their chemical structure because silicone rubbers of this type are hydrophobic, i.e. they are water-repellant. Monomers and oligomers separate from these rubber types and remain behind in the nozzle outlet area of the printing head after removal of the sealing material form the area. The nozzle outlet area or portion thereof exhibiting these deposits can, thereafter, no longer be wetted by the ink spread on this area. At the nozzle rim and in the proximity of the nozzle, the symmetry and thus the uniformity of the droplet formation is disturbed. These disturbances lead to an altered breakup characteristic of the droplets and thus to an impairment of the image reproduction on the recording medium. It has been found experimentally that it is impossible to completely suppress these deposits, and the addition of hydrophilic additives to the covering material is not feasible due to the hydrophobic character of the silicone.